In the commercial cracking of gas oils to produce hydrocarbons boiling in the motor fuel range, there are a wide variety of reactions which take place, some of which are desirable, and some of which are undesirable. It is believed that the refiner today strikes a compromise between the desirable reactions and the undesirable reactions which are taking place, and the basis of such compromise varies from situation to situation and from refiner to refiner. It is simple to state that a refiner desires to make the maximum amount of desirable products, and yet to accomplish this in an economically favorable manner. However, when the above simple statement is buttressed against economic reality, compromises must be made. Thus, for example, a process which yields the maximum amount of gasoline from a barrel of crude oil is not necessarily the most economically attractive one at any given location, for the simple reason that such a process might require a catalyst which must be subjected to more frequent regenerations, or said process might simply bear too heavy an economic penalty as compared to a process which produces slightly less gasoline from a given barrel of crude, but at cheaper cost. Thus, the typical refiner of today is constantly compromising between the most effective manner of carrying out a desired reaction and the practicality of such desired reaction based on such ever-changing factors as the nature and type of the crude being used, the particular market value of the various products which can be obtained from the crude, the cost and nature of the catalyst which is being used, the availability and constraints caused by existing equipment, and quite recently, by restraints imposed by the Environmental Protection Agency with respect to certain emission standards. It also appears quite obvious that these various compromises will become even more troublesome in the future due to the dependency upon foreign crude and the fact that the most desirable feed for a particular operation may not be available or may be available at too high a cost, such that careful balances must be struck between obtaining the maximum amount of useful products from a barrel of crude so as to minimize our dependency upon foreign crude and accomplishing the same as to optimize the economic aspects of the operation and comply with the various regulations with respect to pollution primarily with respect to the nature and extent of contaminants which are released to the atmosphere.